A mathematical model was developed to calculate internal forces in the thumb. The solution was indeterminate, since the number of unknown forces exceeded the equations available in the static force analysis. To resolve the problem, the muscle forces were obtained in conjunction with quantitative electromyographic data. The relationship between muscle force and integrated electromyograms (IEMG) was obtained in terms of polynomial function. A multiple-polynomial regression analysis of the measured force and IEMG data was used to determine the correlation coefficients. Then the muscle forces were calculated with the thumb performing tip, key, and pulp pinches and grasp. The muscle forces were found to be in the range of 3-120 N for the flexor pollicis longus and 4-40 N for the extensor pollicis longus.
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